The invention relates to the field of active pixelated transparent optical elements, in particular to carry out an ophthalmic lens.
Within the meaning of the invention, an optical element is transparent when an object that is located on a first side of the optical element can be viewed without significant loss of contrast by an observer who is himself across the optical element. The object and the observer are each located at distance of the optical element. In other words, an image of the object is formed through the optical element with no significant loss of quality of visual perception for the observer, also called “the wearer”.
It is known to carry out an optical element in the form of a transparent substrate which supports on at least one of its faces, a set of juxtaposed cells that covers at least in part this face. Substances having specific optical properties are contained in the cells, and cooperate to give optical characteristics required for a particular application to the optical component. For example, transparent substances having different refraction index can be divided in cells, so that the resulting component is a draft of lens adapted to correct visual defects. The optical properties regarding how the wavefront is modified by the optical element, also called the “dioptric function”, result in the optical combination of the transparent substrate and of the set of juxtaposed cells.
The juxtaposed cells can be in the form of a film which can be adhered on the optical transparent substrate.
A final lens can then be obtained by edging the optical element according to a contour that corresponds to a frame chosen by the wearer.
Such a transparent optical element comprising a set of cells juxtaposed parallel to a surface of the optical element is generally called a pixelated optical element.
Such a transparent optical element can also have various additional optical functions, such as light absorption, polarizing capability, reinforcement of contrast capability, etc. . . .
The dioptric function of the optical element can be characterized by an optical phase-shift distribution for a given monochromatic light wave which crosses the optical element.
In a general way, the transparent optical element has a surface which extends transversely compared to an optical axis. An average direction of propagation of the light wave can then be selected to be superimposed on this axis, and the optical phase-shift distribution can be given inside the said surface of the element. In case of pixelated optical elements, optical phase-shift has discrete values which are carried out in points which constitute a sampling of the usable surface of the transparent optical element. In a simplified way, optical phase-shift could be well established in a zone limited around each point of sampling, usually called cell. The value of the optical phase-shift of the element in any point of each cell would be thus equal to that of the point of sampling which is located in this cell. In a more realistic way, phase-shift is not constant inside each cell, but is intermediate between a minimal value and a maximum value which are fixed by a target function of phase-shift for this cell. The cells are contiguous in the usable surface of the optical element, and form a paving of this surface. The actual dioptric function of the pixelated transparent optical element results then from the combination of this paving with the values of optical phase-shift which are carried out in all the cells. Contiguous pixels can be separated by a wall having a width.
Moreover, it is well-known that optical phase-shift Δφ for a monochromatic light wave is equal to the product of the double of number pi by the length of crossing L of each cell, and by the difference between the value n of refraction index of the transparent material which fills this cell and the value of the air index and by the inverse wavelength λ. In other words: Δφ=2π*L*(n−1)/λ. A way of carrying out the transparent optical element can then consist in varying the refraction index value of fill material of the cells between different cells of the element. In this case, all the cells can have the same depth, which is measured according to the optical axis of the element.
For example, for the corrective lens application, it is advisable for different cells of the optical element to contain substances of varying refraction index such that the refraction index is adapted to vary along the surface of the optical element, according to the estimated ametropy of an eye to be corrected.
Nevertheless, one can note that optical defects, such as parasitic images generated in case of periodical repartition of pixels or blurring in case of non-periodical repartition of pixels (Voronoï structure detailed below), may appear when wearing pixelated transparent optical elements.